Back-pressure control in pressure drilling



Allg. 15, 1939. G, BAYS 2,169,675

BACK-PRESSURE CONTROL IN PRESSURE DRILLING Filed Dec. 20, 1958 Patented` Aug. l l 5, 19,39-

BACK-PRESSURE CONTROL r IN PRE SSUR DRILLING George S. Bays, Tulsa, Okla., assignor to Stanolind Oil and Gas Company, Tulsa, Okla., a corporation oi,Delaware Application December 20, 1938, `Serial No. 246,864

6 Claims. (Cl. Z55-24) This invention relates to a system of handling drilling fluids in connection with pressure drilling operations. More particularly it relates to a method and apparatus for controlling back 5 pressure in such operations.

In drilling wells, particularly oil and gas wells, it has' long been customary to use a drilling fluid, usually a so-called drilling mud, to carry away the cuttings, cool and lubricate the bit, build wall l and prevent the intrusion of unwanted fluids into the well. More recently drilling operations have been carried out in which the drilling fluid is under a pressure in excess of that corresponding to the hydrostatic pressure of the fluid column.

l Such pressure drilling operations are particularly important in areas where heaving shale is encountered, especially in areas where the pressures encountered in a bore hole shift from that detei-mined by the hydrostatic head to that congg trolled by the overburden or overlying earth pressure. It is my belief that this shifting of pressure is sometimes quite abrupt, especially rin Gulf Coast areas, in which case the pressure within less than 100 feet can change from one 25 based on approximately 470 pounds per 1000 feet in depth to one more than double that ligure.

However this may be, pressure drilling operations are very useful to obtain a bottom hole mud pressure suiliciently high to mud olf a cav- 30 ing or heaving formation without resorting to the use of excessive amounts of weighting materials; to keep oil, water or gas from coming into the hole; to bring in a well without sealing oli production with heavy drilling fluids; and to 35 drill or bring in a well with a controlled ilow, thus preventing initial velocities in a producing formation which might break down the walls of the well and cause excessive caving or heaving.

v The conventional method of circulating mud, 40 oil or other drilling fluid is down through the drill pipe and up through the annular space between the drill pipe and the casing although reverseI The drilling fluid` Moreover, high pressure settlers It has therefore been considered necessary in many pressure drilling operations to maintain the back pressure on the drilling fluid by the use of reducing valves or chokes through which the drilling fluid is vented to' atmospheric pressure. The mud then flows into the conventional settling pits or is otherwise handled at atmospheric or other relatively low pressure to remove cutT tings and then picked up at this low pressure by the circulating mud pumps and reinjected into the well. However, in using this system under Gulf Coast conditions I have found that even b-y using both vibrating screens and centrifuges for the elimination of cuttings,A these cuttings cannot be eliminated satisfactorily. Apparently the mud passing through a choke across which its pressure is reduced from, for instance, 1,000 pounds per square inch to atmospheric, is subjected to a very severe shearing action which converts the shales or clays into a finely divided or colloidal state. This raises the viscosity of the mud so-high that it cannot be'handled eiliciently and at the same time greatly lowers itsA weigh or density which is often artificially obtained by the use of expensive weighting materials. Thisv loss of weight is due to the failure to remove the shale cuttings from the mud and since these cuttings are of lower density than the average density of the mud there is a continual reduction in mud weight.

For these reasons I have found itessential either when the back pressure is controlled by an enclosed circulation system with pressure separation of cuttings or when the back pressure is controlled by the use of chokes, to control the weight andviscosity of the drilling mud by'continuously adding water to reduce viscosity and adding weighting materials to increase the specic gravity. Not only is this highly expensive but I have found it practically impossible. Thus in a specific case, under Gulf Coast conditions, it was found that the weight of the mud could not be increased appreciably beyond pounds per gallon and with the back pressure'limited to 1,000 pounds per square inch it was calculated that a l21/ pound mud was required to hold back the heaving shale. Not being able to obtain a mud weight above 10 pounds, the heaving shale finally closed in on the drill pipe, sticking-it and losing the hole- The colloidal clay and shale troubles encountered in pressure drilling operations inmcertan Gulf Coast and other areas is not due to the high mud pressures used but rather to the shearing f action which takes place when the pressure is -miliar to those skilled in' the art.

suddenly reduced across a choke. In other words, when the pressure is continuously and gradually reduced through the normal ilow of the mud from about 1000 pounds per square inch to atmospheric no trouble is encountered in removing the cuttings. v

It is an object of my invention to provide new and improved methods of bacl-rV pressure control in pressure drilling operations. Another object of my invention is to provide apparatus for this purpose. A moreV detailed object of' my invention is to provide methods for the more complete removal of cuttings from drilling fluids used in pressure drilling operations. A further object of my invention is to provide drilling fluid handling systems in which the cuttings can be removed more completely and satisfactorily than has heretoforebeen the case and in which this is done without substantial loss of4 the energy .available by virtue of the pressure)` of the drilling uid coming from a pressure drilling operation.

My invention will be described with particular reference to the accompanying drawing which is a simplified diagrammatic elevation, partly in section, of one form of apparatus in which my invention can be carried out.

In brief my invention contemplates the use of an hydraulic motor as a back pressure regulator. The high pressure drilling fluid is used to operate such a motor and in the course of this operation its pressure is reduced without subjecting any appreciable partr of the drilling fluid` to any shearing action since it enters the cylinder or motor chamber at high pressure and leaves at low pressure without any sudden pressure drop across an orifice as occurs when the pressure is reduced across a valve or choke.

Turning more particularly to the drawing, the well being drilled is not shown in its entirety but v 'only the parts which are important from the have been omitted from the drawing. Above them the upper end of the kelly 1 (or the drill pipe itself` if no kelly is used) is connected to swivel 8 through whichdrilling mud under high pressure is introduced from the discharge of mud pump 9 by means of line I and flexible hose II.

Drilling mud thus introduced may be at a pressure which will depend upon many factors fa- Generally speaking, the pressure at which the drilling fluid is introduced will range from about 1500 to about 5000 pounds per square inch, typically about 3000 pounds per square inch. This drilling uid passes down through the drill pipe to the base of the well and then up through the annular space between the drill pipe and the casing. However, my' invention is equally applicable to a system in which the circulation is maintained in the other direction, i. e. so-called reverse circulation. The

drilling fluid at a back pressure of from about' 500 to about 2000 pounds per square inch, for instance about 1000 pounds per square inch. passes from the annular space between tubing 4 and casing I out through line I2 which is preferably en'- tirely free of chokes and other obstructions across any` substantial pressure drop would occur and thence to a pressure reducer I3 which can be described as'a pump with two fluid ends I4 and I5, end I4 running in reverse, i. e. as an hydraulic motor, the` mud entering the cylinder at high pressure and doing work on the piston throughout the working stroke, atl the end of which the cylinder is closed by a mechanical lvalve and pressure admitted to the opposite side of the piston, the mud which has already done its work being expelled to the atmosphere.

` Thus pressure reduction chamber or motor end I4 of the pump takes in a cylinder full of high pressure drilling duid through a suitable valve and discharges it at low pressure through another valve into lineV I6. If the uid contains gas the inlet valve can be closed well before the power stroke is completed to permit expansion but in any event no more than a minuscule portion of the fluidis subjected to any substantial pressure drop across the exhaust valve. In pump I2 the pressure is reduced, typically, from 1000 pounds per square inch to atmospheric or other low pressure and the drilling mud at this low pressure passes to mud tank, pit or other mud settling chamber II'in which the cuttings settle out. It is, of course, possible to place a vibrating screen I8 or other cutting separator between pump I3 and mud pit I'I if desired.

Power made available by the expansion of the drilling mud in the power or motor end I4 of pump I3 can beutilized in various ways. Thus for instance, fluid end I canbe used to pick up mud from the mud tank or pit I1 through line I9, act as a mixer and recirculate the mixed mud to the pit through valve 20 and line 2|. However, this fluid end I 5 operated by the high pressure drilling mud in motor end I4 can be made of small size relative to end I4 and can be utilized to boost the pressure on a small part of 'the drilling mud to the pressure at which it is introduced through line I0 and hose Il. .This can be accomplished by opening valves 22 and 23 and closing valves 20 and 2,4. Thus the energy made available by the expansion of the drilling mud is used to pump a quantity of drilling mud to the higher pressure used for injection and fluid end I5 then operates in parallel with the main mud pump 9.

A still further method of using this energy is to close valves 20, 23 and 25 and open valves 22 and 24, thus using the iluid end I5 of pump I2 in series with the main mud pump 8, powered by "steamV or other external power, to handle the total amount of mud reinjected into the well.

The system of my invention need not be con- .nned to the use of a reciprocating type motor I4 to reduce the drilling fluid pressure, as either single or multiple stage rotary gear or displace-Y ment pumps or other prime 'movers might be substituted as hydraulic motors, the main object being to release the pressure of the drilling fluid without the use of a shearing choke, since pressure release is necessary to remove the drill cuttings eillciently.

' While I have described my invention in connection with certain speciflc embodiments thereof, it is to be understood that these are by Way of illustration and not by way of limitationv and I do not mean to be bound thereby but only to the scope of the appended claims which should be construed as broadly as the prior art will permit.

I claim: 1.1In a pressure drilling operation in which drilling fluid comesfrom a well at a high pressure carrying cuttings, the steps which comprise reducing the pressure on said drilling fluid to apsaid drilling fluid into said settling chamber at;

proximately atmospheric pressure without subjecting it to any substantial shearing action, removing cuttings from the drilling fluid at approximately atmospheric pressure, and reinjecting said drilling fluid, after removal of said cuttings, into said well at a high pressure.

`2. In a pressure drilling operation in which drilling fluid comes from a well at a high pressure carrying cuttings, the steps which comprise reducing the pressure on said drilling fluid by causing it to operate an hydraulic motor, removing cuttings from the drilling fluid at' approximately atmospheric pressure, and reinjecting said drilling fluid, after removal of said cuttings, into said well at a high pressure.

3. In a pressure drilling operation in which drilling fluid comes from a well at a high pressure carrying cuttings, the steps which comprise reducing the pressure on said drilling fluid by causing it to do mechanical work, removing cuttings from the drilling fluid at approximately atmospheric pressure, reinjecting said drilling fluid, after removal of said cuttings, into said well at a high pressure, and using said mechanical work t'o operate an auxiliary mud pump.

4. In pressure drilling apparatus comprising a line for drilling fluid under pressure leading from a well, a drilling fluid settling chamber and.

means for reinjecting said drilling fluid from said settling chamber into said well, the improvement which comprises a prime mover arranged to be .operated by the drilling fluid under pressure flowing through said line and adapted to discharge greatly reduced pressure without subjecting it to any substantial shearing action.

5. In pressure drilling apparatus comprising a line for drilling fluid under pressure leading from a well, a drilling fluid settling chamber and means for reinjecting said drilling fluid from said settling chamber into said well, the improvement which comprises a prime mover arranged to be operated by the drilling fluid under pressure flowing through said line and adapted to discharge said drilling fluid into' said settling chambcr at greatly reduced pressure without subjecting itto any substantial shearing action, and an auxiliary drilling fluid pump operated by said prime mover and arranged to recirculate drilling fluid from and to said settling chamber.

6. In a pressure drilling apparatus comprising a line for drilling fluid under pressure leading from a Well, a drilling fluid settling chamber and means for reinjecting said drilling fluid from said settling chamber into said well, the improvement which comprises a prime mover arranged to be operated by the drilling fluid under pressure flowing through said line and adapted to discharge said drilling fluid into said settling chamber at greatly reduced pressure Without subjecting it to any substantial shearing action, and an. auxiliary drilling fluid pump operated by said prime mover and arranged to assist said -means for reinjecting said drilling fluid from said settling chamber into said well.

' GEORGE S. BAYS. 

